Date: Mon, 31 Mar 2014 11:25:20 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Andres Freund <andres@2ndquadrant.com>
Cc: Stephen Frost <sfrost@snowman.net>, pgsql-hackers@postgresql.org
Subject: Re: MultiXactId error after upgrade to 9.3.4
Message-ID: <20140331142520.GY9567@eldon.alvh.no-ip.org>
References: <20140330040029.GY4582@tamriel.snowman.net>
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 <20140330185349.GD4582@tamriel.snowman.net>
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 <20140331115713.GA18358@alap3.anarazel.de>
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Alvaro Herrera wrote:
> Andres Freund wrote:
> > On 2014-03-31 08:54:53 -0300, Alvaro Herrera wrote:
> > > My conclusion here is that some part of the code is failing to examine
> > > XMAX_INVALID before looking at the value stored in xmax itself.  There
> > > ought to be a short-circuit.  Fortunately, this bug should be pretty
> > > harmless.
> > > 
> > > .. and after looking, I'm fairly sure the bug is in
> > > heap_tuple_needs_freeze.
> > 
> > heap_tuple_needs_freeze() isn't *allowed* to look at
> > XMAX_INVALID. Otherwise it could miss freezing something still visible
> > on a standby or after an eventual crash.
> 
> I think this rule is wrong.  I think the rule ought to be something like
> "if the XMAX_INVALID bit is set, then reset whatever is there if there
> is something; if the bit is not set, proceed as today".  Otherwise we
> risk reading garbage, which is what is happening in this case.

Andres asks on IM: How come there is garbage there in the first place?
I have to admit I have no idea.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services


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